1. Field of the Invention
The present invention relates to a ring network formed by nodes and, more particularly, to a path modification system used in such a ring network to modify the used path without interrupting the main signal by checking the quality of the transmission line.
2. Description of the Prior Art
One conventional path modification system is now described. It is assumed that a first line has been already set up from a first node to a second node. The first node is so set up that a second line extends from outside the ring to a third node. The same SDH (synchronous digital hierarchy) signal is sent to the second line as done to the first line. The third node is so set up that the SDH signal received from the first node is sent to a fourth node. The fourth node receives the SDH signals from the first and second lines, respectively, at the second and third nodes, respectively. The fourth node is set up in the manner described below to switch the direction of reception of signals to be sent to the outside of the ring.
The line going from the second node to the outside of the ring is disconnected at the fourth node. The fourth node is so set up that a signal received at the third node is sent to the outside of the ring. To remove the setting on the side of the first line used prior to the modification, the line setting at the second node to go from the first node to the fourth node is deleted. The modification of the path from the first line to the second line is completed by the procedure described thus far.
The operation of the conventional method of modifying the path as described above is described below. The second node is receiving the signal that is sent from the first node to the first line. This state is referred to as state 1. To change the path, the first node transmits the same signal to the second line as done to the first line. This state is referred to as state 2. The second node disconnects the line receiving from the first line (state 3) and connects the line receiving from the second line (state 4). Subsequently, the first node stops input of signals to the first line. Thus, modification of the path is completed.
It is not assured that the line set up to make a transition from state 1 to state 2 is transmitting the signal to be transmitted to the first line. That is, the line set up to make a transition from state 1 to state 2 is merely a novel line added. Since the second node cannot receive two different signals simultaneously, it is necessary to disconnect the first line connected to the second node in order to make a transition from state 2 to state 4. The second node connects the second line. Because the second node does not have a function of checking the quality of the signals received from the first and second lines, respectively, and their identity, the signal from the second line is received on the assumptions that the novel second line added to the first node is transmitting the same signal as done by the first line and that the second line has good quality. However, the conventional path modification system has the following problems.
First, both paths are once broken at the fourth node and so the path cannot be modified during operation. Secondly, it is impossible to check at the fourth node that the signals passing through the first and second lines are the same and, consequently, there is a possibility that a signal different from the signal sent prior to the modification of the path is transmitted. Thirdly, the quality of the second line that will be used is not checked at the fourth node and so a connection may be made to a line that is poorer in quality than the first line used heretofore. In consequence, the line may be broken.
It is an object of the present invention to provide a path modification system which having various functions, e.g., BRDG registration setting, BRDG deletion, ROLL setting, and quality checking, and which is capable of changing a used path in a transmission network system from a line in which quality deterioration occurs during operation of the system to a line of high quality, and which is capable of preventing occurrence of misconnection that would normally be caused by selecting a different signal.
The above-described object is achieved by a path modification system in accordance with the present invention, the system being used to modify the used path in a transmission network system formed by nodes. This path modification system comprises a quality checking means for checking the quality of a transmission line and a bit error checking means for checking bit errors of a received signal. The path is modified so that the main signal is not interrupted.
Other objects and features of the invention will appear in the course of the description thereof, which follows.